Climate change and global warming is considered one of today's the most pressing and severe environmental problems. It is now generally accepted that the main cause for global warming is the release of so-called greenhouse gases into the atmosphere. A major greenhouse gas is carbon dioxide (CO2), which is released predominantly from combustion of fossil fuels such as coal, petroleum and natural gas. Together, these fossil fuels supply about 80% of the energy needs of humanity. Because fossil fuels are still relatively inexpensive and easy to use, and since no satisfactory alternatives are yet available to replace them on the enormous scale needed, they are expected to remain our main source of energy for the foreseeable future.
One way to mitigate CO2 emissions and their influence on the global climate is to efficiently and economically capture CO2 from its point sources, such as from the emissions of fossil fuel-burning power plants and various industrial factories, naturally occurring CO2 accompanying natural gas, and the air, and then to sequester or convert the obtained CO2 to renewable fuels and materials.
Among various CO2 collection or separation techniques, amine solution-based CO2 absorption/desorption systems are still one of the most suitable for capturing CO2 from high volume gas streams. Commonly used solvents in such systems are aqueous solutions of alkanolamines such as monoethanolamine (MEA), diethanolamine (DEA), diisopropanolamine (DIPA), and methyldiethanolamine (MDEA). Certain sterically hindered amines, such as 2-amino-2-methyl-1-propanol (AMP), can also be used as absorbents because of their high CO2 loading capacities. Of these, MEA is most widely used because of its high CO2 absorption rate, which allows use of shorter absorption columns. The MEA system presents major drawbacks, however, including the large amount of heat required to regenerate the aqueous solution and operational problems caused by corrosion and chemical degradation. To prevent excessive corrosion, typically only 10 to 30 weight % MEA is used in an aqueous amine solution, with the rest being water. Because the entire solution, of which 70 to 90% is water, must be heated to regenerate the MEA system, a large amount of energy is wasted during the regeneration process. Other alkanolamine systems also present disadvantages. For example, secondary and hindered amines (e.g., DEA, DIPA, AMP) provide more moderate CO2 absorption rates than MEA, and are also prone to corrosion and chemical degradation. MDEA is known to absorb CO2 only at a slow rate. Formulations formed by blending several alkanolamines are of interest because they can combine favorable characteristics of various compounds while suppressing in part their unfavorable characteristics. A number of blended alkanolamine solutions have been developed, and the most common blends are MDEA-based solution containing MEA or DEA. However, blended alkanolamine solutions do not eliminate the drawbacks of amine solution-based systems.
CO2 can also be captured by adsorption on solid sorbents. Solids are typically used as physical adsorbents for separation of CO2. Such processes are based on the ability of porous solids to reversibly adsorb certain components in a mixture. The solids can have a large distribution of pore size, as in silica gel, alumina, and activated carbon, or a pore size controlled by the crystal structure, e.g., shape selective zeolites. At low temperatures such as room temperature, zeolite-based adsorbents have high CO2 absorption capacities (e.g., 160 mg CO2/g for zeolite 13X and 135 mg CO2/g for zeolite 4A at 25° C. in pure CO2). However, the adsorption capacities of these adsorbents decline rapidly with increasing temperature and in the presence of water or moisture. Further, because gases are only physically adsorbed on the adsorbents, actual separation of an individual gas from a mixture of gases is low.
To achieve a higher selectivity for CO2 adsorption, a compound providing chemical absorption can be applied on the solid adsorbent. For this purpose, an amine or polyamine can be deposited or grafted onto a solid support. Amines and polyamines chemically bound (grafted) on the surface of solids, such as silicas and alumina-silicas, however, show in general limited adsorption capacity of less than 90 mg CO2/g and, in most cases, less than 50-60 mg CO2/g absorbent under dry conditions (Choi, S. et al., ChemSusChem, 2, 796-854, (2009)). For example, U.S. Pat. No. 5,087,597 to Leal et al. discloses a method for chemisorption of CO2 at room temperature using silica gel having a surface area between 120 and 240 m2/g, which is modified with a polyalkoxysilane containing one or more amino moieties in its structure. The material is disclosed to be capable of absorbing between 15 and 23 mg of dry CO2 per gram of absorbent. U.S. Pat. No. 6,547,854 to Gray et al. discloses a method for preparing amine-enriched sorbents by incorporating the amine onto the surface of oxidized solids. The reported maximum amount of CO2 absorbed on these solids is reported to be 7.7 mg/g absorbent using a gas mixture of 10% CO2 in Helium. As is evident from the data, the amount of CO2 that can be absorbed on the grafted amino group on various solid supports remains relatively low, because of their low amine coverage. Hyperbranched amino silica in which aziridine is polymerized directly off the surface of the silica offers somewhat higher amine content and higher CO2 adsorption capacity (Hicks, J. C. et al., J. Am. Chem. Soc. 130: 2902, 2008).
Another pathway involves impregnating a solid support with amines or polyamines. For example, a paper by S. Satyapal et al., J. Energy and Fuels 15:250 (2001) describes the development of polyethylenimine (PEI)/polyethylene glycol (PEG) on a high surface area polymethylmethacrylate polymeric support. This solid was developed to be used in spacecrafts to remove CO2 from the cabin atmosphere and release it into space. Its capacity is approximately 40 mg CO2/g absorbent at 50° C. and 0.02 atm. CO2. This material and its modifications are disclosed in U.S. Pat. Nos. 6,364,938; 5,876,488; 5,492,683; and 5,376,614 to Birbara et al. The preferred supports described in these patents are of polymeric nature, with acrylic ester resins such as AMBERLITE® being described as having particularly suitable characteristics. U.S. Pat. Nos. 5,376,614; 5,492,683; and 5,876,488 also disclose other possible supports, including alumina, zeolite and carbon molecular sieves. According to U.S. Pat. Nos. 5,492,683 and 5,376,614, however, the amount of amine present on the sorbent is limited, ranging from 1 wt. % to 25 wt. %.
U.S. Pat. No. 4,810,266 to Zinnen et al. discloses a method for creating CO2 sorbents by treating carbon molecular sieves with amine alcohols. This patent discloses that monoethanolamine (MEA)-based materials are not stable and release MEA during the regeneration step at higher temperatures. International Publication No. WO 2004/054708 discloses adsorbents based on mesoporous silica supports. The active components for CO2 adsorption are amines or mixture thereof chemically connected or physically adsorbed on the surface of the mesoporous silicas. Adsorption on most of the adsorbents described in this publication is below 70 mg CO2/g. The best results are obtained by using diethanolamine (DEA), which is physically adsorbed on the support (about 130 mg CO2/g). However, because of the volatility of DEA under the desorption conditions, the effectiveness of this adsorbent generally decreases with increasing number of CO2 adsorption-desorption cycle (a decrease of about 16.8% after 5 cycles at a moderate regeneration temperature of only 60° C.). U.S. Pat. No. 6,908,497 to Sirwardane et al. discloses a method for preparing sorbents by treating a clay substrate having a low surface area of 0.72 to 26 m2/g with an amine and/or ether.
Alcohols, polyethylene glycol and other oxygenated compounds have also been used for decades for acid gas removal, mainly CO2 and H2S. For example, SELEXOL® from Union Carbide (now Dow Chemicals) and SEPASOLV MPE® from BASF are used in commercial processes. Oxygenated compounds in combination with amines as mixed physical or chemical sorbents, in a process such as a glycol-amine process, have also been used for many years for acid gas removal (see Kohl, A. L. and Nielsen, R. B., GAS PURIFICATION 5th ed. 1997, (Gulf Publishing Co.)). U.S. Pat. No. 4,044,100 to McElroy demonstrates the use of mixtures of diisopropanolamine and dialkyl ethers of a polyethylene glycol for removing gases, including CO2 from gaseous streams. The use of ethylene glycol to improve the absorption and desorption of CO2 from amines has also been studied by J. Yeh et al., Energy and Fuels 15, pp. 274-78 (2001). While the literature mainly relates to the use of amines and oxygenated compounds in the liquid phase, the use of oxygenated compounds to improve characteristics of gas sorbents in the solid phase has also been explored. S. Satyapal et al., Energy and Fuels 15:250 (2001) mentions the use of polyethylene glycol in conjunction with polyethyleneimine on a polymeric support to remove CO2 from the closed atmosphere of a space shuttle. X. Xu et al., Microporous and Mesoporous Materials 62:29 (2003) shows that polyethylene glycol incorporated in a mesoporous MCM-41/polyethyleneimine sorbent improves the CO2 adsorption and desorption characteristics of the tested material. Preparation and performance of a solid adsorbent consisting of PEI deposited on a mesoporous MCM-41 is also disclosed (see X. Xu et al., Energy and Fuels 16:1463 (2002)). U.S. Pat. Nos. 5,376,614 and 5,492,683 to Birbara et al. use polyols to improve adsorption and desorption qualities of the adsorbents. Improvements were also noticed by Goeppert et al. (Energ. Environ. Sci. 3:1949-1960, (2010)) and Meth et al. (Energ. Fuel. 26: 3082-3090(2012)).
Another new material for trapping carbon dioxide are metal organic framework compounds. A preferred compound known as MOF-177 (J. Am. Chem. Soc., 2005, 127, 17998) has a room temperature carbon dioxide capacity of 140 weight percent at a relatively high pressure of 30 bar.
Yet another adsorbent for this purpose is a supported amine sorbent comprising an amine or an amine/polyol composition deposited on a nano-structured support, which provide structural integrity and increased CO2 adsorption capacity. This material is disclosed in U.S. Pat. No. 7,795,175. The support for the amine and amine/polyol compositions is composed of a nano-structured solid. The nano-structured support can have a primary particle size less than about 100 nm, and can be nanosilica, fumed or precipitated oxide, calcium silicate, carbon nanotube, or a mixture thereof. The amine can be a primary, secondary, or tertiary amine or alkanolamine, aromatic amine, mixed amines or combinations thereof. In an example, the amine is present in an amount of about 25% to 75% by weight of the sorbent. The polyol can be selected from, for example, glycerol, oligomers of ethylene glycol, polyethylene glycol, polyethylene oxides, and ethers, modifications and mixtures thereof, and can be provided in an amount up to about 25% by weight of the sorbent.
Despite these prior disclosures, there still remains a need for an improved sorbent for capturing CO2, which is efficient, economical, readily available and regenerative, and which provides a high removal capacity at ambient as well as elevated temperatures.
Instead of adding polyols and amines based sorbents to enhance the CO2 adsorption/desorption properties, the alcohol groups could be chemically bound to the amines and polyamines. One of the possibilities is to react epoxides with the amino groups of these amines and polyamines. In fact this reaction is commonly used in many applications for the curing of so-called “epoxy resins” where an epoxy resin is reacted with an amino compound (epoxy hardener or curing agent). The two components are generally mixed just before use. Application are numerous and include the formation of adhesives, primers for paints, coatings, production of molds, laminates, castings, fixtures and others. Each primary amino group is theoretically capable of reacting with two epoxide groups, and each secondary amine group is capable of reacting with one epoxide group. The reaction of a primary amine with an epoxide leads to a secondary amine which can itself react further with an additional epoxide to form a tertiary amine, as shown in FIG. 1.
To obtain optimum properties in the product, the curing agent (amine) and epoxide are generally reacted in stoechiometric quantities. To be more precise, the amount of amine N—H bonds is chosen to be equivalent or close to the amount of epoxide groups in the epoxy resin, so that all the amine N—H bonds and epoxide groups would react to form a solid.
The formation of a solid where all the amine N—H bonds would have reacted to form mostly tertiary amines would not result in the most efficient CO2 adsorption characteristics. Thus, these prior art material do not disclose or inherently provide desirable CO2 adsorption.
The reaction of an amine with an epoxide increases the molecular weight of the obtained compounds resulting in a lower volatility. This is particularly important for relatively low molecular weight amines such as for example diethylenetriamine (DETA) triethylenetetramine, (TETA) and tetraethylenepentaamine (TEPA) which have a tendency to leach out when impregnated on solid support as was shown in a number of papers (Qi, G. et al. Energy Environ. Sci. 2012, 5, 7368; Liu, S.-H. et al. Adsorption 2012, 18, 431.; Yan, W. et al. Ind. Eng. Chem. Res. 2012, 51, 3653.; Wang, W. et al. Energy & Fuels 2013, 27, 1538; Qi, G. G. et al. Energy Environ. Sci. 2011, 4, 444.). When epoxides containing several epoxide groups (2, 3 or more) are used, crosslinking can occur between amines.
The crosslinking of amines with epoxides for the purpose of capturing CO2 has been described to some extent. Andreopoulos et al. (Polymers Advanced. Technol. 1991, 2, 87-91) describes the impregnation of polyethylene fibers with polyethylenimine (PEI, Mw-50000-60000)/epoxy resin (Epon 828). The CO2 adsorption capacity obtained was, however, very low, most likely due to the poor surface area of the support. There is no mention of recyclability of the adsorbent. The solvents used for the preparation of the adsorbents were methanol, acetone and dimethylformamide (DMF) which are not benign.
Li et al. (J. Appl. Polym. Sci. 2008, 108, 3851) coated PEI (Mw˜25000)/epoxy resin (Bisphenol A epoxy resin) on a glass fiber matrix and obtained higher CO2 adsorption capacities than the ones reported by Andreopoulos et al. The presence of moisture had a significant positive effect on the adsorption capacity. The solvents used for the preparation for the adsorbent were methanol and DMF.
Gebald et al. (WO 2010/091831 A1) also described the preparation of adsorbents based on fibrous materials on which crosslinked amine was impregnated. The crosslinked amine was the result of the reaction of an amine with an epoxy resin. The authors only described the reaction of two types of amines, i.e. PEI and TEPA, and one type of epoxy resin, i.e. D.E.R. 332, a bisphenol A diglycidylether manufactured by Dow Chemicals. The solvent used for the preparation of the adsorbent was ethanol.
A sorbent based on PEI and D.E.R. 332 on carbon fiber lead to an adsorption capacity of 56.8 mg CO2/g adsorbent from a gas mixture containing 500 ppm CO2 and 100% humidity at 20° C. At 50% relative humidity a much lower adsorption capacity of only 12.5 mg CO2/g was obtained. Reacting TEPA with D.E.R. 332 gave an adsorbent with an adsorption capacity of 82.5 mg CO2/g with a 50% relative humidity at 20° C. The PEI based adsorbent was tested for recyclability in three consecutive adsorption/desorption cycles during which, the adsorption capacity remained similar. On the other hand the adsorbent based on TEPA was not submitted to a similar treatment and recyclability was therefore not demonstrated. In the example for the preparation of TEPA/epoxy resin containing sorbent, the amount of epoxy resin was 0.55 g or 0.00161 mol. The amount of TEPA used was 5 g or 0.02641 mol. Even taking into account that the epoxy resin (D.E.R. 332) has two epoxide groups in each molecule able to react with amino groups, the molar ratio of TEPA/epoxide groups is still only 8.2 (amino groups/epoxide groups ratio of 41). Ideally a ratio of 1 or lower would be necessary for all TEPA molecules to react with at least one epoxide group. This means that due to this high ratio, a majority of the TEPA added at the beginning of the reaction is probably still present, unreacted, in the adsorbent material. The volatility problem of TEPA and other low molecular weight amines have been presented vide supra. It is therefore likely that the adsorbent based on TEPA described in this patent suffers from some leaching problems. Interestingly, the weight of the TEPA based adsorbent during TGA analysis dropped by only 28.5% when heated up to 750° C., which was significantly lower than the 50% expected, indicating some possible loss of TEPA during the preparation itself.
Peiffer et al. (U.S. Pat. No. 8,557,027) described the preparation of epoxy- amine materials for the purpose of CO2 adsorption. The obtained materials were, however, not impregnated or deposited on any support. Their adsorption capacity seemed therefore quite limited at ambient to moderate temperatures (25-50° C.) and they exhibited their highest, although still limited, adsorption capacity at around 80-110° C. This implies that the CO2 desorption would require even higher temperatures and/or a combination with lower pressure, meaning a higher energy input during the desorption. Furthermore, when porogens were used during the preparation to increase the surface area, an additional step of extraction of these porogens with solvents was required at the end of the synthesis.
Meiller (U.S. Pat. No. 4,112,185) described the preparation of an ion exchange resin based on modified porous materials with their surface covered with a cross-linking polymer resulting from the reaction of a polyamine with an epoxide. However, the epoxide compound was generally added in excess (by weight) compared to the amine compound, leading most probably to the formation of a large proportion of tertiary amines. While this resulted in materials with suitable properties for ion exchange application they would have had limited activity for CO2 capture.
Considering the state of the art described here, improvements to the prior art materials is therefore now needed. The present invention now addresses the deficiencies of the prior art to provide new materials as well as their preparation on suitable supports for use in CO2 adsorption.